1. Field of the Invention
The present invention relates to electronic supply systems for fluorescent tubes with electrodes and, more particularly, to electronic supply systems for fluorescent tubes with electrodes that allow virtual instantaneous ignition and energy savings.
2. Description of the Prior Art
The majority of fluorescent tubes currently in use are supplied by a system which has several shortcomings. Two major shortcomings are high energy consumption and rapid tube deterioration. These prior art assemblies comprise a reactance and use the opening of a bimetallic strip to energize a gaseous mixture located between filaments in the tube. Because ignition is rarely instantaneous, several openings of the bimetallic strip are generally required. These repeated openings cause the tube to flicker, which is detrimental to its service life. Further, the reactance consumes a large amount of energy which causes it to heat. Still further, the reactance introduces a large amount of reactivity into the mains line, which increases consumption proportionately.
Research has been carried out with a view towards producing a supply system using only semi-conductor components and transformers so as to consume very little energy and so as to allow instantaneous ignition.
One design, among others, is illustrated in French Specification No. 2,520,575. This design uses two series-mounted transistors which amplify a square signal supplied by an auxiliary frequency generating circuit and transmit it to a series oscillating circuit comprising a coil L1, a lighting unit and a charge capacitor C3.
This system was rapidly improved by omitting the related frequency-generating circuit and by replacing it with a transformer which receives a sinusoidal signal from the oscillating series circuit and uses it to control the two transistors. This system, which is simple in design and is illustrated in French Specification FR No. 2,478,933, has two disadvantages. First, its start-up is random and second, short periods during which the transistors simultaneous allow passage of current are possible. During such periods, the lighting unit is extinguished as it no longer receives a supply and the transistors are traversed by high intensity current which is harmful to their service life.
Various patents providing solutions to the above-mentioned problems have been filed. Three of them, specifications EP No. 0171108, DE No. 3,412,944 and WO No. 87/00719, are discussed in some detail below.
Certain start-up of a circuit is attained, for example, by transmitting a starting pulse to one of the transistors. This is effected by means of a breakdown element, or diac, connected on the one hand to the base of the transistors whose emitter is grounded and on the other hand between a capacitor and a resistor connected in series between the supply terminals and constituting a time base. Thus, once the voltage of the capacitor terminals is higher than the sum of the emitter to base voltage of the transistor and the breakdown voltage of the diac, the diac allows the starting pulse to pass through. However, as the charging capacitor of the LC series circuit is generally grounded, it is imperative that the series circuit has been charged before the transistor which short-circuits it allows current to pass due to the pulse from the diac. This requires an additional system which allows the series circuit to be precharged during the voltage increase of the time constant capacitor. This complicates the general circuit. Thus, a resistor 51 short-circuits the transistor 11 in specification EP No. 0171108; a special arm comprising a diode 17 and parallel connection of a capacitor 19 and resistor 21 controls a switch 14 allowing a capacitor 15 to charge through a capacitor 18 in specification WO No. 87/00719; and a weak charging current passes through a diode 31 and a resistor 32 in specification DE No. 3,412,944.
The problem of overlap in the conducting time of the bipolar transistors is caused by the fact that the time for passage from the saturated state to the block state is directly dependent on the collecter to emitter intensity. This phenomena implies, on one hand, thermal instability in the transistors and, on the other hand, a rapidly attained limit with regard to the number of lighting units with the base system. To overcome these disadvantages, it is necessary to use systems which are more complex for synchronizing the state of the power transistors so as to prevent simultaneous circulation of current in them which would lead, in the worse case, to a breakdown of those elements. This problem is solved in the European specification by means of a complex control circuit comprising time unit elements such as capacitors 33 and in specification WO No. 87/00719 by arranging several transistors in parallel, each one being controlled by its own secondary winding. The system described in specification DE No. 3,412,944 can apparently accommodate only one lighting unit.
A device has also been described in French Specification No. 2,487,140, which achieves certain objectives in a more elegant manner but which is used specifically for discharge lamps which do not include electrodes. This device generates a frequency of at least 0.5 mHz which is required for such discharge lamps. This device cannot be used for fluorescent tubes which operate optimally at a frequency between 100 and 200 kHz. Furthermore, the form of the AC voltage is detrimental to the service life of the coils. Moreover, this system does not allow several lighting units to be connected simultaneously. Finally, the yield of this device is only 87 lm/W, whereas the device proposed by the applicant allows a yield of 125 lm/W to be achieved.